Before checking the distance between the rails, there is also the very critical step of laying the first rail down perfectly straight. For this step I g-clamp short pieces of wood to the tips of the rails so that the wood protrudes out the side of the rail to form "shelf-brackets". Then lay a reference straight edge (extruded alu) on these "shelf-brackets" so the rail and the straight edge lie parallel and snug against each other. Then use feeler gauges to feel for gaps between the straight edge and the rail.

I agree completely, Gerald. But I would consider a different method - somewhat akin to the "MadVac" strategy. The problem with using pieces of wood clamped to the rail is that there is no way to know that they are in alignment. I would go so far as to say that they will not be in alignment and any attempts at using a short reference straight edge will not help. 10'/3 m straight edges are available, but would cost more than the entire mechmate!!

I think my method would be more accurate over longer distances and would be orders of magnitude cheaper. It also will index to the milled point of the rail and not to the surface of the angle iron.

First, you need to have two gauge blocks milled our of 3/4" square stock as in the attached drawing:

The gauge has two opposing v-grooves with the depths of the grooves exactly 0.125" apart - the deeper fits over the top of the rail and the shallower carries and indexes a piece of highly tensioned piano wire. The gauge blocks have been drilled and tapped for set screws to hold them in position at the very ends of the rails.

In this way, you could use a piece of 0.125" thick material (aluminum, etc.) to act a gauge between the piano wire and the rail, shimming and bolting as you go down the rail.

If you wanted to get really clever, you could make the gauge blocks out of something non-conductive like UHMW poly or bakelite and energize the rail and wire with 6 or 12 volts DC and use a conductive gauge between them - wire a light or buzzer in series. In this setup, you would adjust the distance between the rail and wire until the light/buzzer just went out/off.

Sorry Gerald, I misunderstood and thought you were using the blocks of wood as spacers!

However, it got me to thinking that my little gauge blocks should be modified with another v-groove to allow for straightening the rail in both directions:

Using the single piano wire stretched above the point of the rail would only allow a visual "comparison" for straightening in the "y" direction. The addition of a second tensioned wire in the additional v groove would allow the use of a gauge block/feeler to set it more precisely. If you were to check the straightness/flatness of a 3m piece of aluminum bar stock, you would be amazed at how 'un'straight it is. A length of 0.020 piano wire, tensioned to 50 kg would be much more accurate - at least over such a short distance where gravity shouldn't be too much of a factor.

In the woodworking industry, extruded alu sections are the most common reference straight lines. The way to check the straightness is to turn it over (and end_for_end) and repeat the alignment procedure. If nothing changes after turning the extrusion around/over then it is a good reference.

I didn't have any extruded aluminum around the garage when it came time to bolt down my X rails and ensure they were straight so I came up with an alternative strategy.

I began by clamping a strip of metal to the vertical part of the end of one the main beams. Then I clamped one to the other end. Halfway up the strips, I added a small clamp. Then I stretched some .010" fishing line between the clamps immediately over the top of the center of the bevel. The threads of the small clamp are great for holding the line in place. If the threads are too coarse and the line needs to move slightly one way or another, you can simply bend the metal strip a little bit.

Once the line was in place over each end of the rail, I used a 5X jewellers loupe to look down on top of the line and was easily able to see where adjustments needed to be made.

For those that want to take the use of stretched wires to a high level of precision, consider that old-time millwrights set up massive machines using piano wire and a few precision tools (like precision gauge blocks and various styles of indicators). One of the important things to know is the effect of gravity on horizontally stretched wire. The link below has "sag" tables: